When two nouns come together (like “school bag”), they form a noun compound. Noun compounds are used very often in Turkish. In a noun compound, the first element is the possessor, and the second one is the possessed. In Turkish, the possessed noun in an indefinite noun compound takes a suffix. This is the same with the possessive suffix for the third person singular (“his/her/its”).

öğrenci >> öğrenci + si >> üniversite öğrencisi “university student”

If the word ends in a consonant, the letter s is omitted from the suffix, as explained in possessive suffixes.

otobüs >> otobüs + ü >> okul otobüsü “the school bus”

adam >> adam + ı >> iş adamı “businessman”

If the possessed noun is in plural, it takes the possessive suffix for “their”.